Claude Sonnet 5.5 (effort default (no --effort flag) · thinking on) on Make an ignore-file pattern matcher follow its spec (anchoring, directory-only, double-star, negation, parent exclusion)

AClaude Sonnet 5.5default · thinking on · Claude Code 2.1.299 (headless -p) · cloud · 2026-10-20 · by @bobsiopener

Trust 80%Fully autonomoussuccess · first tryCompare

Estimated cost: $0.13
Wall time: 55s
Human step-ins: 0
Outcome: Pass

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Exact prompt

# Make the ignore-file matcher follow its spec `ignore.py` decides which paths our sync tool skips, using `.gitignore`-style patterns. It is a quick `fnmatch` hack and gets many cases wrong. Rewrite it so `Matcher(text).ignored(path, is_dir=False)` follows the rules below exactly. Python 3 standard library only. Keep the class name, constructor and method signature. `path` is a relative POSIX path with no leading `./` or `/` (e.g. `src/app/main.py`); `is_dir` says whether the path itself is a directory. Every ancestor of `path` (e.g. `src`, `src/app`) is a directory. ## Rules 1. **Lines.** Split `text` into lines (strip a trailing `\r`). Trailing spaces are removed unless the last space is escaped as `\ ` (backslash-space, which keeps one literal space). Blank lines are ignored. A line starting with `#` is a comment; `\#` at the start is a literal `#`. Leading spaces are NOT stripped. 2. **Negation.** A leading `!` makes the pattern a negation (re-include). `\!` at the start is a literal `!`. 3. **Directory-only.** A trailing `/` is removed and makes the pattern match only directories. A pattern that is empty after removing `!` and the trailing `/` is ignored. 4. **Anchoring.** If the remaining pattern contains a `/` anywhere (start or middle), it is matched against the full path from the root (a leading `/` is dropped first). Otherwise it is matched against the last component at any depth (i.e. against the full path or any suffix that starts right after a `/`). 5. **Wildcards.** `*` matches any run of characters except `/` (including none); `?` matches exactly one character except `/`. `[...]` is a character class: ranges like `a-z` work, `!` or `^` as the first character negates it, a `]` directly after `[` (or after the negation mark) is literal; a class never matches `/`. A `[` with no closing `]` is a literal `[`. A backslash outside a class makes the next character literal. 6. **Double star.** Only these forms are special: a leading `**/` matches zero or more leading directories; `/**/` matches one `/` or `/any/dirs/`; a trailing `/**` matches one or more characters (anything inside, but not the directory itself). Any other run of `*` (e.g. `a**b`, `**` on its own) behaves like a single `*`. 7. **Last match wins.** For a given path, look at all patterns in file order; the last one that matches decides (normal = ignored, negated = not ignored). No match = not ignored. Directory-only patterns are only considered when the path being tested is a directory. 8. **Parents.** First test each ancestor directory from the top (`src`, then `src/app`, ...) using rule 7 with `is_dir=True`. If any ancestor is ignored, the path is ignored and nothing (not even a later `!` pattern) can re-include it. Otherwise the path's own result from rule 7 is the answer. 9. **Speed.** Building a matcher from 300 patterns and answering 20,000 `ignored()` calls must take under 10 seconds in total on our (shared, slow) test box. ## Scoring Hidden checks (one per group, all must pass; partial credit is the fraction passed): `comments_spaces_escapes` (rules 1-2, 5 backslash), `negation_last_match` (2, 7), `dir_only` (3, 7), `anchoring` (4), `wildcards_classes` (5), `double_star` (6), `parent_exclusion` (8), `randomized_agreement` (400 random pattern/path pairs built from the characters `a b / * ? [ ] ! -` must all agree with the rules), `speed` (9). Each group has 10-40 exact assertions.

Nine interacting rules (escaped trailing spaces, class edge cases like []] and [!]], three special ** forms, last-match-wins, and 'a parent directory once excluded cannot be re-included') checked by exact assertions plus 400 randomized pattern/path pairs, so half-right regex translations score partially. Original benchmark task written for usecaserank.com (not based on a specific public post). Headless, offline run on a reproducible fixture. Each run's notes say how its cost was measured (estimates are marked).

Success criteria: 1. comments_spaces_escapes 2. negation_last_match 3. dir_only 4. anchoring 5. wildcards_classes 6. double_star 7. parent_exclusion 8. randomized_agreement (400 random pairs) 9. speed (300 patterns x 20,000 paths < 10 s)

Setup notes: Use Case Scout batch 8 case i01-gitignore-matcher (offline fixture). Prepared with batch8/prepare.sh; hidden key/tests kept outside the agent's work dir. Prompt = TASK.md verbatim. Scored with the scout's scorer: `python score.py WORKDIR KEYDIR`. Tools: Read/Write/Edit/Glob/Grep/Bash, web disabled. Single run, no retries. 1-min load average at scoring: 2.14. Claude Code's own reported cost for claude-sonnet-5.5: $0.128523. Score: 9/9. Cost is an ESTIMATE: measured tokens x pricing.json list price (Claude Pro subscription, nothing billed); input_tokens include cache reads and writes. Single run.

Estimated cost List-price estimate; not actually billed.

Metrics

Estimated cost: $0.13
Wall time: 55s
Human step-ins: 0
Turns: 4

Model usage
Model	Input tok	Output tok	Cost
AClaude Sonnet 5.5anthropic/claude-sonnet-5.5effort default (no --effort flag) · thinking on	90,017	6,773	$0.13

Reproduce this run

Use the use case prompt on the use case page.
Same setup: Claude Sonnet 5.5 (anthropic/claude-sonnet-5.5, effort default (no --effort flag) · thinking on) via Claude Code 2.1.299 (headless -p), cloud.
It counts as a success only if it meets every success criterion: 1. comments_spaces_escapes 2. negation_last_match 3. dir_only 4. anchoring 5. wildcards_classes 6. double_star 7. parent_exclusion 8. randomized_agreement (400 random pairs) 9. speed (300 patterns x 20,000 paths < 10 s)
Log model, tokens, wall time, human step-ins and the evidence log, then submit it as a reproduction. A different model? Submit a head-to-head rerun.

How runs are scored: Methodology.

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Automated checks

Trust 80% comes from these checks (how Trust % works).

Run date is not in the future : passed
Model is in the dated pricing table : passed
Cost recomputed from tokens matches : passed
Cached tokens are consistent : passed
Output throughput is plausible : passed
Autonomy matches step-ins : passed
Step-ins do not exceed turns : passed
Retries agree with first-try result : passed
Evidence log SHA-256 matches : passed
Evidence log is substantial : passed

Evidence log

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    "provider": "firstParty",
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  "permission_denials": [],
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  "result": "I rewrote `ignore.py` to follow the nine rules, and it passes the spot checks I wrote. I didn't have your hidden checks, so I haven't confirmed the groups pass.\n\n- **What I ran:** about 20 assertions covering escapes, negation with parent exclusion, directory-only patterns, anchoring, the double-star forms, character classes and a literal `[`. All pass.\n- **Speed:** 300 random patterns and 20,000 `ignored()` calls took about 0.02 seconds. Those patterns were random strings, so they may match less than your real ones. The matcher also caches results per path and per ancestor directory, which helps when paths repeat.\n- **How it works:** each pattern is turned into a regular expression. All the patterns are joined into one expression, tried last-to-first, so the first alternative that matches is the last matching pattern in file order. That is how rule 7 is applied.\n- **Randomized-agreement group:** I did not run a randomized comparison against a separate reference implementation. Inputs that the spec leaves open are the most likely place for a mismatch.\n\nWhere the spec was ambiguous, I made these choices:\n- **Leading `/` before `**`:** the leading `/` is dropped first. So `/**` becomes a bare `**`, which acts as a single `*`, not as a \"match everything inside\" form.\n- **Overlapping `**` forms:** these are read left to right. In `**/**`, the leading `**/` is special and the second `**` is a single `*`.\n- **Backslash inside `[...]`:** it is treated as an ordinary character. A reversed range such as `[z-a]` matches nothing.",
  "ttft_ms": 2752,
  "type": "result",
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  "uuid": "[REDACTED]",
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  "first_content_frame_ms": 2329,
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 }
]